US 7548011 B2
Methods and apparatuses for sensing and adjusting lateral motion in a comb drive actuated MEMS device are provided. If lateral motion is sensed by a lateral motion sensor coupled to the comb drive actuated MEMS device, and the lateral motion is greater than a reference value, a feedback controller adjusts the lateral motion by providing a drive signal to a comb drive electrode of a comb drive actuator.
1. A method comprising:
sensing lateral motion associated with a comb drive actuated MEMS device;
providing a value corresponding to the sensed lateral motion to a feedback controller;
comparing the sensed lateral motion value with a reference value;
providing a signal if the sensed lateral motion value is greater than the reference value, the signal being provided from the feedback controller to a comb drive electrode; and
adjusting the lateral motion to substantially zero using the signal.
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7. An apparatus, comprising:
a comb drive actuated MEMS device;
a plurality of lateral motion sensors coupled to the comb drive actuated MEMS device for sensing motion in a lateral direction;
a feedback controller coupled to the lateral motion sensors for adjusting lateral motion in the comb drive actuated MEMS device, the feedback controller being configured to provide a drive signal to a comb drive electrode coupled to the comb drive actuated MEMS device.
8. The apparatus of
9. The apparatus of
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11. The apparatus of
This invention was made with government support under grant No. DAAD19-02-1-0366 awarded by the Army Research Office and grant No. GOALI BES 0201773 awarded by the National Science Foundation. The government has certain rights in this invention.
1. Field of the Invention
The present invention relates generally to Micro-Electrical-Mechanical System (MEMS) technology. More particularly, it concerns improving the useful traveling range of MEMS actuators.
2. Description of Related Art
Micro-electrical-mechanical systems (MEMS) often use actuators to impart motion to, for example, positioning optical devices and switches and for turning gears. Electrostatic actuators are one of several types of actuators used in MEMS, including magnetic piezoelectric, thermal, and optical actuators. When compared to other micro actuators, electrostatic actuators generate force of several micro Newtons (μN) and consume virtually no electrical power.
One of the most common electrostatic actuators is a comb drive, which generates a force dependent on the square of the applied voltage. The main issue of the comb drive design is achieving large deflections while minimizing the actuation voltage, resulting in a small deflection-to-size ratio of the actuator. These requirements are typically met by balancing the design of the actuator's suspension and varying the size of the force-generating comb structure. However, comb drives inherently suffer from an electromechanical instability called side pull-in or lateral instability, as illustrated in
To overcome this, the transverse stiffness of the suspension is generally increased. Unfortunately, current suspension stiffening techniques limit the traveling range of the actuator. For example, fabrication techniques such as deep reactive ion etching (DRIE) have allowed the comb structure to be fabricated in single crystal silicon with typical thicknesses of several tenths of microns. These thicker structures can provide larger vertical electrode areas and substantially higher stiffness, but do not improve the limited deflection of comb drive structures.
The shortcoming of conventional methodologies are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques concerning MEMS actuators. Other noteworthy problems may also exist, however, those mentioned here are sufficient to demonstrate the methodologies appearing in the art have not been altogether satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.
The present disclosure provides, amongst other things, techniques to minimize or substantially eliminate lateral motion. In one respect, lateral motion in a comb drive actuated MEMS device is sensed and a value corresponding to the lateral motion is provided to a feedback controller. The sensed lateral motion value is compared to a reference value. If the sensed lateral motion value is greater than the reference value, a signal is provided from the feedback controller to a comb drive electrode, and adjusting the lateral motion to substantially zero using the signal.
In other respects, an apparatus is provided. The apparatus may include, amongst other components, a comb drive actuated MEMS device, a plurality of lateral motion sensors, and a feedback controller. The lateral motion sensors may be coupled to the comb drive actuated MEMS device for sensing motion in a lateral direction. The feedback controller, coupled to the lateral motion sensor, may adjust lateral motion in the comb drive actuated MEMS device. In one embodiment, the feedback controller may be configured to provide a drive signal to a comb drive electrode coupled to the comb drive actuated MEMS device.
The term “coupled” is defined as connected, although not necessarily directly, and not necessarily mechanically.
The terms “a” and “an” are defined as one or more unless this disclosure explicitly requires otherwise.
The term “substantially” and its variations are defined as being largely but not necessarily wholly what is specified as understood by one of ordinary skill in the art, and in one non-limiting embodiment “substantially” refers to ranges within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5% of what is specified.
The terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”) and “contain” (and any form of contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a method or device that “comprises,” “has,” “includes” or “contains” one or more steps or elements possesses those one or more steps or elements, but is not limited to possessing only those one or more elements. Likewise, a step of a method or an element of a device that “comprises,” “has,” “includes” or “contains” one or more features possesses those one or more features, but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way is configured in at least that way, but may also be configured in ways that are not listed.
Other features and associated advantages will become apparent with reference to the following detailed description of specific embodiments in connection with the accompanying drawings.
The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The figures are examples only. They do not limit the scope of the invention.
The disclosure and the various features and advantageous details are explained more fully with reference to the nonlimiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well known starting materials, processing techniques, components, and equipment are omitted so as not to unnecessarily obscure the invention in detail. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the invention, are given by way of illustration only and not by way of limitation. Various substitutions, modifications, additions, and/or rearrangements within the spirit and/or scope of the underlying inventive concept will become apparent to those of ordinary skill in the art from this disclosure.
The present disclosure provides modeling of a comb actuator and using feedback control to manage axial and lateral motion. In one respect, a novel comb actuator structure with built-in sensors may be provided and allow for feedback control of movements. In one respect, the sensor can sense and distinguish lateral from axial motion, which provides a more effective control of comb drive actuators including control of lateral instability. Additionally, the control system may impact the design of the comb drive, mitigating the requirements on the suspension, lowering the actuation voltage and therefore, decreasing the ratio between size of the actuator and achievable deflection.
Modeling Comb Capacitor Unit (Capacitance and Force)
Both generated force and the sensitivity of the comb structure may depend on the capacitance. Therefore, a model of one basic comb capacitor unit, as shown in
The partial derivatives of the capacitance of Eq. 1 with respect to x and y are given as follows
It is noted that the MEMS device shown in
For the purpose of illustrating how feedback stabilizes lateral motion of the device, a model is derived, where the parameters of the lateral degree of freedom (DOF) model are analyzed using a combined finite element analysis (FEA) and static experimental results. Use of a model to design a controller does not affect the ability of the controller to stabilize lateral motion. However, the lateral motion significantly contributes to the simplicity and straightforwardness of the controller design.
In one embodiment, the device shown in
It can be seen that θ is small during the operation of the device. In the other words, a y-direction movement of the comb structure is smaller than the radius of rotation, i.e. y<<LA. Hence, the approximation of tgθ=y/LA≈θ may be assumed, and thus, modifying Eq. 8 to
Forces Fx and Fy from Eqs 7 and 9 may be determined as a contribution of all forces generated by comb capacitors, such as one shown in
For typical MEMS application, it may be desirable that lateral y force from Eq. 11 does not exist (e.g., Fy=0). However, due to various imperfections, Fy generally is not equal to zero. Lateral instability occurs if lateral force becomes large enough to overcome lateral stiffness.
One of the reasons for unsymmetrical lateral force is unsymmetrical geometry. To model this, a virtually symmetrical comb drive actuator is modeled as an unbalanced one by introducing Δd, as shown in
In order to determine total forces Fx and Fy from Eqs. 7 and 9, the procedure from Eqs. 1-5 may be used. The capacitances CF and CB are composed of two comb capacitance units as the one shown in
With the capacitances from Eqs. 12 and 13, the force in the x direction, Fx, and the force in the y direction, Fy are calculated in a similar fashion as they were in Eqs. 4 and 5 for the drive unit shown in
The lateral comb-like structures in
The lateral actuators can include top and bottom comb drive structures designed to generate force in the y direction. These comb drive structures are unbalanced with unequal gaps (a and b) between the electrodes, as shown in
The lateral sensor has a similar gap geometry in order to achieve maximum sensitivity. The number of fingers, N, and the initial electrode overlapping, x0, may be different than with the lateral actuator. Movable capacitors are connected to the bridge structure through serial capacitors. Deflection in the y-direction can be determined from the difference between voltages.
Similar to the unbalanced comb actuator on the MEMS device shown in
The total force in the lateral direction, Fya, may be calculated using a similar procedure as in Eqs. 14 and 15
A differential change in capacitance can be sensed by a properly designed voltage amplifier coupled between VsT and VsB shown in
A sense bridge is designed so that all motion except motion in the y direction (or around the z axis) is rejected. Motion in the x direction may be cancelled by adding symmetrical structure pointing into the negative direction of x. Eqs. 19 and 20 show this embodiment clearly. Any out of plane motion affects both the top and bottom part of the sensor in the same way, therefore canceling it. Sensitivity may change only for the case when Δy (or y) is relatively large, and this typically does not happen because the controller is assumed to keep lateral motion substantially small.
Feedback Control Design
In one embodiment, the present disclosure provides a feedback approach that prevents lateral pull-in and extends the working range of the comb drive actuator in the primary x-direction DOF. The primary requirement for the controller is to keep movement in the y-direction at substantially zero. Additional shaping of the dynamics may also be desirable.
The structure of an example controller is shown in
In other embodiments, controllers with features for implementing gain scheduling for square voltages may be used. In one embodiment, it is assumed that the lateral motion Δy is measurable and available. When the lateral feedback loop is closed, the sensed value of y is compared to the reference yd=0 and the error signal may be passed through the PID controller. Saturation-type nonlinearities distribute voltages to the two channels (1302 and 1304 of
In order to keep the controller as simple as possible, the lateral degree of freedom can be expressed as:
As noted above, in one embodiment, the controller may be a PID type, with a gain scheduling applied to account for voltage-squared term, i.e. square root from Eq. 30, as given by
A set of simulations are shown in
With the benefit of the present disclosure, those having ordinary skill in the art will comprehend that techniques claimed here may be modified and applied to a number of additional, different applications, achieving the same or a similar result. The claims cover all such modifications that fall within the scope and spirit of this disclosure.
Each of the following references is incorporated by reference in its entirety: